High-pressure discharge lamps are used, for example, in automobiles. In the ignited state they are operated, for example, with the aid of a rectangular signal with a frequency of 400 Hz. Disturbances in the FM band can occur at 70 to 120 MHz if such a signal has steep edges. The disturbances arise on the line, that is to say they are so-called conducted disturbances. Recently, more and more automobile manufacturers have gone over to implementing specific operating elements without direct mechanical or hydraulic connection between the user interface and the action location, instead of supplying for a user an input unit in the case of which the signals input therein are converted into electrical signals that are subsequently transmitted via a bus system to an actuator which then undertakes the appropriate activity. This development is familiar under the term “drive-by-wire”, and is used, for example, in the steering, the brake and the gas pedal of a motor vehicle. It is evident here that no disturbances of any kind may be allowed to occur, since malfunctions could initiate disastrous consequences. In the prior art, an inductor has been introduced for this purpose into the return conductor in the case of circuit arrangements of the generic type for high-pressure discharge lamps. FIG. 1 shows a high-pressure discharge lamp 10, the associated drive circuit being accommodated in the block 12. The input 1 denotes the forward line, input 2 the return line and input 4 the starting line. Arranged inter alia in the block 12 is the starting device. The high-pressure discharge lamp 14, which is cast into a glass cladding 16, has a return line 18 that is insulated by means of a ceramic tube 20. A typical starting voltage is at approximately 23 kV DC, a typical operating voltage is 85 V AC, the frequency being, for example, 400 Hz.
In the known circuit arrangement, conducted disturbances are certainly reliably prevented, but the return conductor voltage of the high-pressure discharge lamp is increased by the inductance. In the event of starting voltages of up to 25 kV, the return conductor voltage can become so high, in particular directly after starting, that it can jump over onto the reflector of the assigned headlight. Because of the high voltages occurring, there is therefore the risk of injury to automobile mechanics or home mechanics who may be accidentally in contact with the reflector when the discharge lamp is switched on. Because of the high temperatures occurring—for example, a typical value of the temperature of the discharge vessel is 700° C., which means that a return conductor in the vicinity is still heated to 550° C.—it is not possible to use any plastic insulations. What are used, for example, are ceramic tubes that for their part must have play, since it is necessary to take account of the expansion at high temperatures and which, on the other hand, can easily break such that the return conductor is entirely unprotected. As shown in FIG. 1, a part of the return conductor itself normally remains entirely uninsulated even given an intact ceramic tube. A further disadvantage of the known solution consists in that the starting voltage provided for the lamp is split between the lamp and the inductor arranged in the return conductor. Again, the starting energy that is available in the lamp for the further starting operation after a flashover caused by the starting voltage is reduced in the prior art. It follows that only a reduced starting voltage and starting energy are available for the lamp, the result being worsening of the starting reliability of the lamp.
It is therefore the object of the present invention to develop a circuit arrangement of the generic type in such a way that the risk of injury is reduced while the required operational reliability is maintained, and a higher starting reliability can be ensured than with the solution known from the prior art.